compiler: Don't crash on invalid print call.

When the print builtins are called with no arguments, the compiler
issues a warning and crashes when trying to produce the backend
representation for the arguments.

Fixes golang/go#11526.

Change-Id: I0616bfdd4f29cedeb44db51a311e706d4628c19c
Reviewed-on: https://go-review.googlesource.com/13131
Reviewed-by: Ian Lance Taylor <iant@golang.org>

go/expressions.cc

@@ -8177,6 +8177,12 @@ Builtin_call_expression::do_get_backend(Translate_context* context)
                                                       location);
 	  }
 
+        // There aren't any arguments to the print builtin.  The compiler
+        // issues a warning for this so we should avoid getting the backend
+        // representation for this call.  Instead, perform a no-op.
+        if (print_stmts == NULL)
+          return context->backend()->boolean_constant_expression(false);
+
         return print_stmts->get_backend(context);
       }